The stainless steel investment casting process can be divided into four stages: wax patterns manufacturing, shell making, smelting pouring and post-processing. We collect the inspection, equipment maintenance and other costs for these four stages into auxiliary production costs. In the four production processes, the costs incurred in the three stages (wax patterns manufacturing, shell making, smelting and pouring) are closely related to the production rate of the process. For example, if the manufacturing cost of wax patterns is calculated according to stainless steel casting weight, the cost relationship between small and large stainless steel investment casting parts is obviously not in line with reality. Therefore, a more reasonable method is to calculate the cost of wax mold manufacturing, shell making, smelting and pouring process according to the weight of pouring stainless steel, and the cost of post-treatment and auxiliary production can be calculated according to the casting weight. The cost of the shell making and smelting stages accounts for more than 60% of the stainless steel investment casting process cost.
Main Factors Affect the Cost of Stainless Steel Investment Castings
Strictly speaking, the manufacturing cost of different stainless steel castings in each process is not completely the same, but the cost differences in some stages are very small and can be calculated according to the average level. What we should pay attention to are the main factors that have a relatively large impact on the cost of stainless steel investment casting. The main factors leading to the cost difference of stainless steel investment casting process are as follows:
1. Process Yield
Process yield is also called yield, is the percentage of actual casting weight in pouring weight. For specific castings, the process yield is equal to the percentage of the total weight of castings in the same tree, which is related to the casting structure and tree combination scheme, and may vary in the range of 30% to 60%, generally between 40% and 50%.
2. Numbers of Shells
Due to the different shape and structure of the stainless steel casting, the number of shells will be different. For example, casting with long and narrow holes or slots needs to be coated with twice or even three layers of shell. It is usually enough to make two overlays for a casting, while for larger castings it may be necessary to make three or more layers. The cost of shell making per kilogram of stainless steel castings was about 1.5usd, of which the cost of materials accounted for 67.8%, fuel and power accounted for 23.9%, wages accounted for 13.3%. In the cost of 1 usd/kg of shell materials, the consumption of zirconium sand zirconium powder is about 63%, accounting for 42.7% of the whole cost of shell making, and the cost of silica sol is about 12.2% of the cost of shell making. Although zirconium sand zirconium powder is only used for surface shell making, it becomes the main item of shell making cost because of its high price.
3. Stainless Steel Casting Grades
Investment casting of stainless steel involves various types of stainless steel grades, such as 300 series, 400 serious, PH17-4, 2205 duplex stainless steel, ect. The cost of each stainless steel casting grade is different, normally, the higher grade of stainless steel, the cost of castings will be higher. The selection of stainless steel grades is mainly decided by the application of castings.
4. Post Treatment
After casting, qualified stainless steel investment castings need to go through such post-treatment processes as broken shell, sand cleaning, cutting, polishing, shot blasting, pickling, shaping, repair welding and refurbishing, etc. The average post-treatment cost can be determined according to the weight of castings, the average post-treatment cost per kilogram of castings is 0.8usd.The pickling cost of stainless steel castings is about 0.1 usd/kg.
The content and difficulty of the post-treatment process are also different when the stainless steel casting structure is different. Generally, it is ok for the stainless steel castings to go through such processes as shell crushing, cutting, grinding and shot blasting after pouring, while some stainless steel investment castings need to be added with additional procedures.The customer requires that the heat treatment, surface treatment, mechanical processing and other work contents in addition to casting blank should be accounted separately into the total price, which is not the scope of this paper.The post-treatment cost difference mainly comes from three aspects: sand cleaning, deformation correction and repair, and its cost depends on the casting structure and technical requirements. The cost difference should be considered when the price is approved.
1) Sand cleaning: Some castings with long and narrow grooves or long and thin holes are difficult to be cleaned, requiring sand drilling, acid biting, sand blasting or alkali blasting to clean. The cost of sand cleaning shall be estimated separately for such castings.
2) Shaping: The deformation of easily deformed castings needs to be corrected. The difficulty of shaping depends on the casting structure, the degree of deformation and customers’ requirements for dimensional and geometric tolerances. The shaping should be accounted separately.
3) Refit: the casting process is a special process, and there are many factors affecting the casting quality. Objectively speaking, it is difficult to avoid surface defects on stainless steel investment castings. Different customers or castings with different uses have different requirements for surface quality. It is very important for both parties to determine a reasonable quality acceptance standard according to casting characteristics and possible surface defects before receiving the order. If the customer for surface quality requirements are higher, repair costs will be higher. The cost of refurbishment is mainly affected by casting quality requirements, which need to be considered in the pricing, and casting primary pass rate, which depend on internal quality control. The cost of refurbishment can be adjusted on the basis of the average post-treatment cost by an appropriate quality rating factor.
5. Apportionment of Administrative Expense
The management fee of a stainless steel foundry also belongs to the stainless steel investment casting cost and can be allocated by weight to specific castings. A preassessment at the time of quotation may determine an expected break-even yield or average yield, at which the management fee per kilogram of casting is calculated.Post – evaluation should be based on the actual amount of distribution.
The management fee is related to the size of the stainless steel foundry and the product structure. A stainless steel investment casting manufacturer with a simple and stable product structure needs a lower management fee, while a foundry with a large variety and a high technical content needs a higher management fee. The management fee for a medium-sized stainless steel foundry is about 1 usd per kilogram of stainless steel castings.